1. Field of the Invention
The present invention relates to an optical disk device to record and reproduce data into and from an optical disk using a light beam, in particular, to an optical disk device to detect disk tilt, that is, a relative tilt angle of the disk recording surface with respect to an optical pickup.
2. Description of the Related Art
Densification of an optical disk can be achieved basically by increasing linear density and reducing track pitches. Also, in an optical system to irradiate the disk with a laser beam, densification and, at the same time, reduction of the beam spot diameter are required. The beam spot diameter is proportional to the wave length of the light source and inversely proportional to the numerical aperture (NA) of the objective lens. Accordingly, it is necessary to shorten the wave length of the light source as densification progresses.
Further, as densification of the optical disk advances, an influence of the disk tilt on recording/reproducing becomes large. Since occurring of a tilt to a disk deteriorates the signal recording characteristic and increases cross talks at the time of reproducing the signal, in an optical disk device to execute recording to media such as DVD-R/RW/RAM and the like, compensation by a tilt servo has been conventionally carried out. This tilt servo controls the optical pickup so that there is no tilt anymore.
Incidentally, the technology to perform the tilt control based on a focus error signal has been disclosed (refer to Japanese Patent Laid-open Application No. 2003-168231, and Japanese Patent Laid-open Application No. 2003-317288).